$ A = \left[\begin{array}{rrr}2 & 0 & 0 \\ 2 & 1 & 1 \\ 1 & 1 & 0\end{array}\right]$ What is $ A^{-1}$ ?
Answer: $ = \left[\begin{array}{rrr}\frac{1}{2} & 0 & 0 \\ -\frac{1}{2} & 0 & 1 \\ -\frac{1}{2} & 1 & -1\end{array}\right]$